comphead commented on code in PR #10501:
URL: https://github.com/apache/datafusion/pull/10501#discussion_r1601777537
##########
datafusion/optimizer/src/push_down_limit.rs:
##########
@@ -217,6 +183,78 @@ impl OptimizerRule for PushDownLimit {
}
}
+/// Combines two limits into a single
+///
+/// Returns the combined limit `(skip, fetch)`
+///
+/// # Case 0: Parent and Child are disjoint. (`child_fetch <= skip`)
+///
+/// ```text
+/// Before merging:
+/// |........skip........|---fetch-->| Parent
Limit
+/// |...child_skip...|---child_fetch-->| Child
Limit
+/// ```
+///
+/// After merging:
+/// ```text
+/// |.........(child_skip + skip).........|
+/// ```
+///
+/// Before merging:
+/// ```text
+/// |...skip...|------------fetch------------>| Parent
Limit
+/// |...child_skip...|-------------child_fetch------------>| Child
Limit
+/// ```
+///
+/// After merging:
+/// ```text
+/// |....(child_skip + skip)....|---(child_fetch - skip)-->|
+/// ```
+///
+/// # Case 1: Parent is beyond the range of Child. (`skip < child_fetch <=
skip + fetch`)
+///
+/// Before merging:
+/// ```text
+/// |...skip...|------------fetch------------>| Parent
Limit
+/// |...child_skip...|-------------child_fetch------------>| Child
Limit
+/// ```
+///
+/// After merging:
+/// ```text
+/// |....(child_skip + skip)....|---(child_fetch - skip)-->|
+/// ```
+///
+/// # Case 2: Parent is in the range of Child. (`skip + fetch < child_fetch`)
+/// Before merging:
+/// ```text
+/// |...skip...|---fetch-->| Parent
Limit
+/// |...child_skip...|-------------child_fetch------------>| Child
Limit
+/// ```
+///
+/// After merging:
+/// ```text
+/// |....(child_skip + skip)....|---fetch-->|
+/// ```
+fn combine_limit(
+ parent_skip: usize,
+ parent_fetch: Option<usize>,
+ child_skip: usize,
+ child_fetch: Option<usize>,
+) -> (usize, Option<usize>) {
+ let combined_skip = child_skip + parent_skip;
Review Comment:
```suggestion
let combined_skip = child_skip.saturating_add(parent_skip);
```
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